Although a broad range of viruses can cause myocarditis, the mechanisms that underlie virus-induced cardiac inflammatory disease are poorly understood. Here, we use mammalian orthoreovirus (reovirus) to examine mechanisms of viral myocarditis. We found that the M2 gene is a determinant of reovirus myocarditis in neonatal mice. The M2 gene encodes viral outer capsid protein μ1, which mediates host membrane penetration during viral entry. A recombinant reovirus in which the M2 gene from strain T3D was substituted into an otherwise strain T1L genetic background (T1L/T3DM2) caused significantly more myocarditis than the parental T1L strain. T1L was non-lethal in wild-type mice, whereas greater than 90% of mice succumbed to T1L/T3DM2. T1L/T3DM2 produced higher viral loads at the site of inoculation and disseminated with more rapid kinetics than T1L. T1L/T3DM2 also produced higher peak viral titers compared to T1L, most notably in the heart where T1L/T3DM2 viral loads were approximately 50-fold higher than T1L. Hearts from T1L-infected animals contained small purulent lesions, consistent with previous reports that T1L is mildly myocarditic. In contrast, hearts from T1L/T3DM2-infected mice were grossly abnormal, with extensive pericardial inflammatory infiltrate. Histological analysis revealed that T1L/T3DM2-infected hearts had larger and more frequent heart lesions containing necrotic cardiomyocytes with pyknotic debris than animals infected with T1L. T1L/T3DM2 also induced more lymphocyte and histiocyte infiltration than T1L. More cells with activated caspase-3 were found in hearts infected with T1L/T3DM2. Flow cytometry profiling of cardiac immune cells revealed that T1L/T3DM2 induced significantly more lymphoid cell (CD4⁺ and CD8⁺T cells) and inflammatory monocyte infiltration than T1L. Together, our findings indicate that the M2 gene, and by extension the μ1 protein, promotes reovirus replication in the heart, leading to increased cardiac inflammation and enhanced myocarditis.